Compliant Layer for Matched Tool Molding of Uneven Composite Preforms

ABSTRACT

A method for consolidating a preform made of composite material. The preform and a compliant metal alloy sheet are placed between less compliant matched confronting forming/molding surfaces with the preform being sandwiched between the metal alloy sheet and a matched confronting surface. The matched confronting surfaces and compliant metal alloy sheet are heated until the preform reaches at least its consolidation temperature. During heating, force is applied so that the matched confronting surfaces exert sufficient compressive force on the preform and metal alloy sheet to cause the composite material to consolidate at the consolidation temperature. The metal alloy sheet has a tensile yield point in a range of 25-300 psi at the consolidation temperature at a strain rate of about 1% to 10% strain per minute.

BACKGROUND

This disclosure generally relates to fabrication of parts made ofcomposite material. More specifically, this disclosure relates toapparatus and methods for consolidating and forming a pre-form made offiber-reinforced plastic material (also referred to herein as acomposite preform) to reduce voids and/or porosity.

Fiber-reinforced organic resin matrix composites have a highstrength-to-weight ratio or a high stiffness-to-weight ratio anddesirable fatigue characteristics that make them increasingly popular asa replacement for metal in aerospace applications. Organic resincomposites may comprise thermoplastics or thermosetting plastics.

Prepregs combine continuous, woven, or chopped reinforcing fibers withan uncured, matrix resin, and usually comprise fiber sheets with a thinfilm of the matrix. Sheets of prepreg generally are placed (laid-up) byhand or with fiber placement machines directly upon a tool or die havinga forming surface contoured to the desired shape of the completed partor are laid-up in a flat sheet which is then draped and formed over thetool or die to the contour of the tool. Then the resin in the prepreglayup is consolidated (i.e., pressed to remove any air, gas, or vapor)and cured (i.e., chemically converted to its final form usually throughchain-extension) in a vacuum bag process in an autoclave (i.e., apressure oven) to complete the part.

In hot press forming, the prepreg is laid-up, bagged (if necessary) andplaced between matched metal tools that include forming surfaces thatdefine the internal, external, or both mold lines of the completed part.The tools and composite preform are placed within a press and then thetools and preform are heated under pressure to produce a consolidated,net-shaped part.

It is known to consolidate and form composite preforms using inductivelyheated consolidation tools. Induction heating is a process in which anelectrically conducting object (usually a metal) is heated byelectromagnetic induction. During such heating, eddy currents aregenerated within the metal and the electrical resistance of the metalleads to Joule heating thereof. An induction heater typically comprisesan electromagnet through which a high-frequency alternating current ispassed. Most organic matrix composites require a susceptor in oradjacent to the composite material preform to achieve the necessaryheating for consolidation or forming. The susceptor is heatedinductively and transfers its heat principally through conduction to thepreform sandwiched between opposing susceptor facesheets. During heatingunder pressure, the number of voids and/or the porosity of a compositepreform can be reduced.

Recycled graphite fibers can be used in the fabrication of compositeaircraft parts, such as lightweight seat back components. First, a matteproduct is fabricated using recycled graphite fibers and virginthermoplastic fibers; then the matte product is consolidated and formedin matched tooling comprising opposing susceptor facesheets. These matteproducts produced with recycled graphite fibers can exhibit a rathermatted and tangled fiber architecture. These products can also haveundesirable unevenness and thickness/density variations. Furthermore,since the graphite fibers are tangled, they do not facilitate the flowof the thermoplastic material. These characteristics lead to theformation of voids and/or porosity in the final composite product due tothe fact that thickness and density distribution are not uniform acrossthe matte product. The formation of voids and/or porosity is furtheraided by the fact that the fibers are entangled and flow is limited andunable to “heal” porosity very effectively. Even prepreg has somevariation in thickness and density distribution, which could lead to theformation of voids and/or porosity during matched tool molding ofprepreg composites.

Accordingly, there is a need for a method and an apparatus that canreduce the number of voids and/or the porosity during the consolidationand formation of a composite preform having uneven thickness and/ordensity distribution.

SUMMARY

The subject matter disclosed herein is directed to a method for reducingthe number of voids and/or porosity during the consolidation andformation of a composite preform having uneven thickness and/or densitydistribution, such as a matte product comprising recycled graphitefibers and virgin thermoplastic fibers. This method is carried out usingan apparatus that comprises matched molding tools. The apparatus furthercomprises a compliant layer that is situated between the compositepreform and one of the matched molding tools for the purpose ofproviding a more even pressure over the entire area of the preformduring the consolidation process. The compliant layer should have anoffset tensile yield point (0.2% of the strain) in a range of 25-300 psiat the temperature of consolidation of the preform at strain rates ofabout 1% to 10% strain per minute.

In accordance with one embodiment, a sheet of magnesium base alloy isused to act as the compliant layer or shim to compensate for uneventhickness or density over the area of a composite preform, for example,a matte product comprising recycled graphite fibers and virginthermoplastic fibers. Magnesium base alloy makes an excellent candidatefor a compliant layer for high-performance thermoplastic resins due tothe fact that some magnesium alloys become very soft at temperaturesuseful for assisting the consolidation and molding of thermoplasticcomposites (i.e., 600-750° F.) and do not melt until above 1000° F. Asthe temperature and pressure increase inside the apparatus duringconsolidation of the composite preform, the magnesium alloy sheetsoftens and forms into the areas of relatively lower pressure. Themagnesium alloy sheet can be reused due to the soft nature of thematerial. Other alloys can be used instead of a magnesium base alloyprovided that the alloy has an offset tensile yield point (0.2% of thestrain) in a range of 25-300 psi at the temperature of consolidation ofthe preform at strain rates of about 1% to 10% strain per minute

In accordance with one aspect, a method is disclosed for consolidating apreform made of composite material with reduced number of voids and/orporosity. The preform and a compliant metal alloy sheet are placedbetween less compliant matched confronting forming/molding surfaces withthe preform being sandwiched between the metal alloy sheet and one ofthe matched confronting surfaces. The matched confronting surfaces areinductively heated (which heats the compliant metal alloy sheet byconduction) until the preform reaches at least a consolidationtemperature of the composite material. During heating, force is appliedso that the matched confronting surfaces exert sufficient compressiveforce on the preform and metal alloy sheet to cause the compositematerial to consolidate at the consolidation temperature. The compliantmetal alloy sheet has a tensile yield point in a range of 25-300 psi atthe consolidation temperature at a strain rate of about 1% to 10% strainper minute.

Another aspect of the disclosed subject matter is an apparatus forconsolidating a preform made of composite material at a consolidationtemperature, comprising: first and second tool assemblies having matchedconfronting surfaces; a metal alloy sheet disposed between said matchedconfronting surfaces, wherein said metal alloy sheet has a tensile yieldpoint in a range of 25-300 psi at the consolidation temperature at astrain rate of about 10% strain per minute; means for heating at leastthe matched confronting surfaces of the first and second toolassemblies; and means for applying force to one or both of the first andsecond tool assemblies so that the matched confronting surfaces arecapable of exerting compressive force on the preform and metal alloysheet.

A further aspect is a method for consolidating a composite preform madeof recycled graphite fibers and organic resin fibers, comprising:placing the composite preform and a metal alloy sheet between matchedconfronting surfaces of first and second tool assemblies, the matchedconfronting surfaces being less compliant than the metal alloy sheet;heating the matched confronting surfaces of the first and second toolassemblies and the metal alloy sheet during a heating cycle; andapplying force to one or both of the first and second tool assemblies sothat the matched confronting surfaces exert sufficient compressive forceon the composite preform and metal alloy sheet to cause thermal couplingof one matched surface to one side of the composite preform, the otherside of the composite preform to the metal alloy sheet, and the metalalloy sheet to the other matched confronting surface. The force isapplied during at least a portion of the heating cycle. The metal alloysheet has a tensile yield point in a range of 25-300 psi at aconsolidation temperature at a strain rate of about 1% to 10% strain perminute.

Other aspects of the invention are disclosed and claimed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will be hereinafter described with reference todrawings for the purpose of illustrating the foregoing and other aspectsof the invention.

FIG. 1 is a diagram showing a sectional view of portions of a knownapparatus, the apparatus comprising upper and lower tool assemblies withmatched surfaces designed to consolidate and form a composite preform.The tool assemblies are shown in their retracted positions and thepreform is shown in an uncompressed state.

FIG. 2 is a diagram showing a sectional view of the apparatus depictedin FIG. 1, except that the tool assemblies are in their extendedpositions with the preform compressed therebetween.

FIG. 3 is a diagram showing an end view of a portion of a lower toolingdie in accordance with one embodiment.

FIG. 4 is a diagram showing a sectional view of a portion of the lowertooling partially depicted in FIG. 3, the section being taken along line4-4 seen in FIG. 3.

FIG. 5 is a diagram showing the placement of an unconsolidated matteproduct between a pair of opposing susceptors in their retractedpositions, the matte product comprising recycled graphite fibers andvirgin thermoplastic fibers.

FIG. 6 is a diagram showing a consolidated matte product between a pairof susceptors in their extended positions.

FIG. 7 is a diagram showing a compliant layer and a consolidated matteproduct between a pair of susceptors in their extended positions.

FIG. 8 is a graph showing the effect of temperature on the tensileproperties of a magnesium base alloy having a chemical composition of2.5-3.5% aluminum; 0.7-1.3% zinc; 0.20-1.0% manganese; balancemagnesium. The 0.2% proof stress curve has been extrapolated to theright of the vertical axis labeled “ELONGATION” to show the anticipatedeffect of temperatures in excess of 300° C. on the 0.2% proof stress.

FIG. 9 is a block diagram showing components of a system comprisingupper and lower tool assemblies with matched surfaces and a compliantmetal alloy sheet disposed therebetween for use in consolidatingcomposite preforms.

Reference will hereinafter be made to the drawings in which similarelements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

The following detailed disclosure describes a method and an apparatusfor consolidating and molding/forming a composite preform having anuneven thickness and/or density distribution. In accordance with thedisclosed method, a compliant layer is placed between the compositepreform and one heated consolidation tool. The compliant layer isdesigned to distribute the molding pressure more evenly over the entirearea of the uneven preform during the consolidation process.

One known apparatus for matched tool consolidation of composite preformsis partly depicted in FIGS. 1 and 2. FIG. 1 shows the apparatus in apre-consolidation stage, while FIG. 2 shows the apparatus whileconsolidation is under way. The apparatus comprises a lower die frame 2,a lower tooling die 4 supported by the lower die frame 2 and having afirst contoured die surface 6, an upper die frame 8, and an uppertooling die 10 supported by the upper die frame 8 and having a secondcontoured die surface 12 which is complementary to the first contoureddie surface 6. The contoured die surfaces 6 and 12 may define a complexshape different than what is depicted in FIGS. 1 and 2. However, thenovel means disclosed herein also have application when the die surfacesare planar. The die frames 2 and 8 may be coupled to hydraulic actuators(not shown in FIGS. 1 and 2), which move the dies toward and away fromeach other. In addition, one or more induction coils (not shown in FIGS.1 and 2) may extend through each of the tooling dies 4 and 10 to form aninduction heater for raising the temperature of the resin in a compositepreform to at least its consolidation temperature. A thermal controlsystem (not shown) may be connected to the induction coils.

Still referring to FIGS. 1 and 2, the apparatus further comprises alower susceptor 18 and an upper susceptor 20 made of electrically andthermally conductive material. The susceptors and the induction coilsare positioned so that the susceptors can be heated by electromagneticinduction. The lower susceptor 18 may generally conform to the firstcontoured die surface 6 and the upper susceptor 20 may generally conformto the second contoured die surface 12. In some cases, it is preferredthat the temperature at which a composite preform is consolidated shouldnot exceed a certain temperature. To this end, susceptors 18 and 20 arepreferably so-called “smart susceptors”. A smart susceptor isconstructed of a material, or materials, that generate heat efficientlyuntil reaching a threshold (i.e., Curie) temperature. As portions of thesmart susceptor reach the Curie temperature, the magnetic permeabilityof those portions falls to unity (i.e., the susceptor becomesparamagnetic) at the Curie temperature. This drop in magneticpermeability has two effects: it limits the generation of heat by thoseportions at the Curie temperature, and it shifts the magnetic flux tothe lower temperature portions, causing those portions below the Curietemperature to more quickly heat up to the Curie temperature.Accordingly, thermal uniformity of the heated preform during the formingprocess can be achieved irrespective of the input power fed to theinduction coils by judiciously selecting the material for the susceptor.In accordance with one embodiment, each susceptor is a layer or sheet ofmagnetically permeable material. Preferred magnetically permeablematerials for constructing the susceptors include ferromagneticmaterials that have an approximately 10-fold decrease in magneticpermeability when heated to a temperature higher than the Curietemperature. Such a large drop in permeability at the criticaltemperature promotes temperature control of the susceptor and, as aresult, temperature control of the part being manufactured.Ferromagnetic materials include iron, cobalt, nickel, gadolinium anddysprosium, and alloys thereof.

The consolidation/molding apparatus shown in FIGS. 1 and 2 furthercomprises a cooling system 14 comprising respective sets of coolingconduits 16 distributed in the tooling dies 4 and 10. Each set ofcoolant conduits 16 is coupled via respective manifolds to a source ofcooling medium, which may liquid, gas or a gas/liquid mixture such asmist or aerosol.

In a typical implementation of a composite consolidation and moldingprocess, the composite preform 22 is initially positioned between theupper and lower tooling dies of the stacked tooling apparatus, as shownin FIG. 1. Then the tooling dies 4 and 10 are moved toward each other,as shown in FIG. 2, as the induction coils heat the susceptors 18 and20. Therefore, as the tooling dies close toward each other, thesusceptors rapidly heat the composite preform 22. During this process,the composite preform will be molded by the opposing contoured (orplanar) surfaces of the susceptors 18 and 20.

After a predetermined interval of time, the cooling system 14 will beoperated to apply a cooling medium to the tooling dies 4 and 10, therebyalso cooling the susceptors 18 and 20 and the composite preform 22therebetween. The composite preform 22 remains sandwiched between thesusceptors for a predetermined period of time until complete cooling ofthe composite preform has occurred. This allows the molded andconsolidated composite preform 22 to retain the structural shape whichis defined by the contoured surfaces of the susceptors 18 and 20. Thetooling dies are then opened and the composite preform can be removed.The formed and cooled composite preform is removed from the stackedtooling apparatus without loss of dimensional accuracy when it is cooledat an appropriate property-enhancing rate.

FIG. 3 is an end view of a portion of a lower tooling die 4 inaccordance with one embodiment. The upper tooling die may have a similarconstruction. Each tooling die comprises a multiplicity of cavities 32,which may be mutually parallel. FIG. 3 shows only two such cavities 32,the upper portion of each cavity 32 having a portion of a respectiveturn of an induction coil 34 which passes through the uppermost portionof the cavity.

The sectional view shown in FIG. 4 is taken along line 4-4 seen in FIG.3 and passes through a cavity 32, but not through the portion ofinductive coil 34 therein. One or more coils can be used. As the partsrequiring fabrication get bigger, it may be necessary to break the coilinto multiple coils connected in parallel in order to limit the voltagerequired by each coil. Without the smart susceptors, control of thecurrent (and resulting temperature) to each parallel coil could becomeproblematic. For the sake of simplicity, FIGS. 3 and 4 show a portion ofa lower tooling die for which the corresponding portion of the attachedsusceptor is horizontal rather than angled.

Still referring to FIGS. 3 and 4, the lower tooling die may comprise alamination of alternating metal (e.g., stainless steel) plates 28 anddielectric spacers 30 which are trimmed to appropriate dimensions toform a plurality of parallel longitudinal cavities 32 in which the turnsof one or more induction coils 34 reside. Each metal plate 28 may have athickness in the range of about 0.0625 to about 0.5 inch. Air gaps 36(see FIG. 4) may be provided between the upper portions of metal plates28 to facilitate cooling of the susceptors. The stacked metal plates 28may be attached to each other using clamps (not shown), fasteners (notshown) and/or other suitable means known to persons skilled in the art.The stacked metal plates 28 may be selected based on their electricaland thermal properties and may be transparent to the electromagneticfield produced by the induction coils.

As best seen in FIG. 4, the smart susceptor 18 is attached directly tothe metal plates 28 of the lower tooling die. (The smart susceptor 20seen in FIG. 1 is likewise attached directly to the metal plates of theupper tooling die.) In accordance with one embodiment, the metal plates28 are made of austenitic (non-magnetic) stainless steel and each plateis 0.185 inch thick, which is suitable when the induction heater isoperated at a frequency of 1 to 3 kHz and with smart susceptors havingthicknesses of 0.125 inch. These lamination plates can be thicker than0.185 inch for lower induction heater frequencies and would need to bethinner when using higher frequencies. These laminations can (andshould) have a space 36 between them to allow the quenching fluid (gasor liquid) to have direct impingement against the surface of the heatedsusceptor 18. This spacing is dictated by the thickness and strength ofthe smart susceptor surface shell and the consolidation pressures used.In addition, the susceptors do not require an electrical connection toone another. The metal plates 28 are interleaved with dielectric spacers30 except near the susceptor and in places that are needed to allow thequenching medium to flow to the susceptor. The same considerations applyto the upper tooling die and the susceptor attached thereto.

Preferably each induction coil 34 is fabricated from copper tubing whichis lightly drawn. A lightly drawn condition of the tubing enablesprecision bending by numerically controlled bending machines.Numerically controlled bending of the tubes allows accurate placement ofthe tubing relative to the changing contours of the susceptors, therebyimproving the degree to which the each susceptor is uniformlyinductively coupled to the induction heater across the length and widthof the susceptor. However, it should be understood that the compliantlayer disclosed hereinafter can be employed also in cases wherein thesusceptors are planar rather than concave/convex. Optionally the coils34 also remove thermal energy by serving as a conduit for a coolantfluid, such as water. After being bent and installed, the coils includestraight tubing sections connected by flexible tubing sections. Theflexible tubing sections connect the straight tubing sections and alsoallow the dies to be separated. The accurate placement of the tubing ofthe induction coils 34 promotes uniformity in the amount of heatgenerated by the magnetic flux field and the amount of heat removed byflow of the coolant fluid.

As disclosed in U.S. Pat. No. 6,528,771, the induction coils 34 can beconnected to a temperature control system that includes a power supply,a controlling element, a sensor and a fluid coolant supply preferablycontaining water (not shown). The power supply supplies an alternatingcurrent to the induction coils 34 which causes the coils to generate theelectromagnetic flux field. The fluid coolant supply supplies water tothe induction coils 34 for circulation through the coils and the removalof thermal energy from the dies. The sensor is capable of measuring thepower supplied by the power supply. Alternatively, or in addition tomeasuring the power supply, the sensor may include a voltmeter that canmeasure the voltage drop across the induction coils 34. The controllingelement receives the sensor output and uses the measurements in afeedback loop to adjust the power being supplied by the power supply.The controlling element can include hardware, software, firmware, or acombination thereof that is capable of using feedback to adjust thevoltage output by the power supply.

The system described with reference to FIGS. 1-4 can be enhanced tofacilitate the processing of composite preforms having uneven thicknessand/or density across the extent of the preform by adding a compliantlayer between the composite preform and one susceptor. For example, itis known to fabricate aircraft parts from a matte product that comprisesrecycled graphite fibers and virgin thermoplastic fibers. FIG. 5 showsthe placement of such a matte product 40 (in an unconsolidated state)between a pair of opposing susceptors 18 and 20 in their retractedpositions. The matte product 40 can have a matted and tangled fiberarchitecture and undesirable unevenness and thickness/densityvariations. Furthermore, since the graphite fibers are tangled, they donot facilitate the flow of the thermoplastic material. As seen in FIG.6, which shows the matte product 40 at the end of the consolidationprocess, these characteristics can lead to the formation of voids 42and/or porosity in the final composite product due to the fact thatthickness and density distribution were not uniform across the matteproduct. The formation of voids and/or porosity is further aided by thefact that the fibers are entangled and flow is limited and unable to“heal” porosity very effectively.

In accordance with various embodiments, the consolidation apparatusfurther comprises a compliant layer that is situated between thecomposite preform and one of the matched molding tools for the purposeof providing a more even pressure over the entire area of the preformduring the consolidation process. The compliant layer should have anoffset tensile yield point (0.2% of the strain) in a range of 25-300 psiat the temperature of consolidation of the preform at strain rates ofabout 1% to 10% strain per minute.

In accordance with one embodiment shown in FIG. 7, a sheet 44 ofmagnesium base alloy approximately 0.125 inch thick is used to act asthe compliant layer or shim to compensate for uneven thickness ordensity over the area of a matte product 40 comprising recycled graphitefibers and virgin thermoplastic fibers. The matte product 40 and themagnesium alloy sheet 44 are shown sandwiched between a pair ofsusceptors 18 and 20 in their extended position, i.e., the moldingapparatus is closed. A magnesium base alloy is selected which becomesvery soft at temperatures useful for assisting the consolidation andmolding of thermoplastic composites (i.e., 600-750° F.) and does notmelt until above 1000° F. As the temperature and pressure increaseinside the molding apparatus during consolidation of the resultingcomposite product, the magnesium alloy sheet 44 softens and forms intothe areas of relatively lower pressure. The magnesium alloy sheet 44 canbe reused due to the soft nature of the material.

More specifically, a suitable magnesium alloy sheet material is ElektronAZ31B Sheet, which is commercially available from Magnesium Elektron UKin Manchester, England. AZ31B is a wrought magnesium base alloy, isnon-magnetic, has high electrical and thermal conductivity, and has amelting range of 1050-1170° F. Superplastic forming of AZ31B can occurduring the preform consolidation process. The chemical composition ofAZ31B magnesium base alloy is: 2.5-3.5% aluminum; 0.7-1.3% zinc;0.20-1.0% manganese; balance magnesium. FIG. 8 is a graph showing theeffect of temperature on the tensile properties of this particularmagnesium base alloy. The 0.2% proof stress curve has been extrapolatedto the right of the vertical axis labeled “ELONGATION” to show theanticipated effect of temperatures in excess of 300° C. on the 0.2%proof stress.

Other magnesium base alloys can be used instead of AZ31B provided thatthe alloy has an offset tensile yield point (0.2% of the strain) in arange of 25-300 psi at the temperature of consolidation of the preformat strain rates of about 1% to 10% strain per minute. Alternatively,metal alloys having a base element different than magnesium, such asaluminum, can be used provided that they have the aforementioned tensileyield property.

A system incorporating a compliant layer 44 the type described above isshown in FIG. 9. In this embodiment, the compliant layer 44 is attachedto the upper tool die 10 by means of clamps, fasteners or other knownmeans (not shown in FIG. 9), with an upper susceptor 20 disposed betweenthe compliant layer 44 and the upper tool die 10. Alternatively, thecompliant layer could be attached to the lower tool die 4 with the lowersusceptor 18 disposed therebetween. During the consolidation process,the upper and lower tool dies are moved toward each other by hydraulicactuators 46, which tool closing motion is indicated by arrows in FIG.9. Electrical power is supplied to the induction coils (not shown) by apower supply 48 in the manner previously described. After consolidationand cooling, the hydraulic actuators 46 move the tool dies apart toallow removal of the consolidated product from the mold. The compliantlayer can be reused.

A compliant layer of the type described above also has application inthe consolidation and forming/molding of composite preforms other thanthe matte product described herein. For example, the compliant layer canbe used in the consolidation of composite preforms that comprisereinforcing fibers embedded in a matrix made of either thermoplastic orthermosetting plastic material.

While the invention has been described with reference to variousembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Forexample, a compliant layer can be used with heated consolidation toolsthat lack susceptors. In cases wherein the tooling dies have heatedmatched surfaces, the compliant layer can be placed between one of thoseheated matched surfaces and the composite preform. In addition, manymodifications may be made to adapt a particular situation to theteachings herein without departing from the essential scope thereof.Therefore it is intended that the claims not be limited to theparticular embodiments disclosed.

The method claims set forth hereinafter should not be construed torequire that the steps recited therein be performed in alphabeticalorder or in the order in which they are recited, and should not beconstrued to exclude two or more steps being performed concurrently.

1. A method for consolidating a preform made of composite material,comprising: placing the preform and a metal alloy sheet between firstand second tool assemblies having matched confronting surfaces which areless compliant than the metal alloy sheet, with the preform beingsandwiched between the metal alloy sheet and one of the matchedconfronting surfaces; heating the matched confronting surfaces of thefirst and second tool assemblies and the metal alloy sheet until thepreform reaches at least a consolidation temperature of the compositematerial; and applying force to one or both of the first and second toolassemblies so that the matched confronting surfaces exert sufficientcompressive force on the preform and metal alloy sheet to cause thecomposite material to consolidate at the consolidation temperature,wherein the metal alloy sheet has a tensile yield point in a range of25-300 psi at the consolidation temperature at a strain rate of about 1%to 10% strain per minute.
 2. The method as recited in claim 1, whereinthe metal alloy sheet is made of magnesium base alloy.
 3. The method asrecited in claim 2, wherein a chemical composition of the magnesium basealloy includes magnesium, aluminum, zinc and manganese.
 4. The method asrecited in claim 1, wherein the metal alloy sheet is made of aluminumbase alloy.
 5. The method as recited in claim 1, wherein the metal alloysheet is very soft at the consolidation temperature.
 6. The method asrecited in claim 1, wherein the composite material comprises a matteproduct.
 7. The method as recited in claim 6, wherein the matte productcomprises recycled graphite fibers.
 8. The method as recited in claim 7,wherein the matte product further comprises thermoplastic fibers.
 9. Themethod as recited in claim 6, wherein the composite material comprisesgraphite fibers and plastic material.
 10. An apparatus for consolidatinga preform made of composite material at a consolidation temperature,comprising: first and second tool assemblies having matched confrontingsurfaces; a metal alloy sheet disposed between said matched confrontingsurfaces, wherein said metal alloy sheet has a tensile yield point in arange of 25-300 psi at the consolidation temperature at a strain rate ofabout 1% to 10% strain per minute; means for heating at least saidmatched confronting surfaces of the first and second tool assemblies;and means for applying force to one or both of the first and second toolassemblies so that said matched confronting surfaces are capable ofexerting compressive force on the preform and metal alloy sheet.
 11. Theapparatus as recited in claim 10, wherein said metal alloy sheet is madeof magnesium base alloy.
 12. The apparatus as recited in claim 11,wherein a chemical composition of the magnesium base alloy includesmagnesium, aluminum, zinc and manganese.
 13. The apparatus as recited inclaim 10, wherein said metal alloy sheet is made of aluminum base alloy.14. The apparatus as recited in claim 10, wherein the metal alloy sheetis very soft at the consolidation temperature.
 15. The apparatus asrecited in claim 10, wherein each of said first and second toolassemblies comprises a respective susceptor, said susceptors formingsaid matched confronting surfaces.
 16. A method for consolidating acomposite preform made of recycled graphite fibers and organic resinfibers, comprising: placing the composite preform and a metal alloysheet between matched confronting surfaces of first and second toolassemblies, the matched confronting surfaces being less compliant thanthe metal alloy sheet; heating the matched confronting surfaces of thefirst and second tool assemblies and the metal alloy sheet during aheating cycle; and applying force to one or both of the first and secondtool assemblies so that the matched confronting surfaces exertsufficient compressive force on the composite preform and metal alloysheet to cause one side of the composite preform to be thermally coupledto one side of the metal alloy sheet, the other side of the compositepreform to be thermally coupled to one of the matched confrontingsurfaces, and the other matched confronting surface to be thermallycoupled to the other side of the metal alloy sheet, said force beingapplied during at least a portion of the heating cycle, wherein themetal alloy sheet has a tensile yield point in a range of 25-300 psi ata consolidation temperature at a strain rate of about 1% to 10% strainper minute.
 17. The method as recited in claim 16, wherein the metalalloy sheet is made of magnesium base alloy.
 18. The method as recitedin claim 17, wherein a chemical composition of the magnesium base alloyincludes magnesium, aluminum, zinc and manganese.
 19. The method asrecited in claim 16, wherein the metal alloy sheet is made of aluminumbase alloy.
 20. The method as recited in claim 16, wherein the metalalloy sheet is very soft at the consolidation temperature.